1. Field of the Invention
The present invention relates to a spark-ignited gas engine with a number of cylinders, wherein on each cylinder a mechanical fuel feed valve for direct feeding of gaseous fuel into the cylinder and a fuel line opening into the cylinder are provided and the fuel line can be closed by the mechanical fuel feed valve in the direction of the cylinder. The invention also relates to a method for controlling such a gas engine and a method for converting a gas engine to such a gas engine.
2. The Prior Art
In spark-ignited gas engines, e.g., large-volume gas engines operated with natural gas in natural gas extraction and transport or in the chemical industry, the gaseous fuel (natural gas, liquefied gas, hydrogen, etc.) is metered at low pressure directly into the cylinders when a mechanical fuel feed valve is opened. When the fuel feed valve is open, gaseous fuel flows through the mechanical fuel feed valve, then through the fuel line and into the cylinder. Because of the low pressure the gas feed takes place at low cylinder pressure, e.g., during the initial compression phase. The fuel feed into the cylinder is discontinued once the fuel feed valve is closed again. The mechanical fuel feed valve is generally controlled by a camshaft and therefore opens for a predetermined crank angle range or a period of time as a function of the speed. On engines that have mechanical fuel injection valves that are operated by a camshaft and push-rods, the valve opens and closes for the same period of time, no matter what the load is, as long as the speed is the same. Since this determines the opening time, the mount of fuel amount fed to the cylinder depends largely on the prevailing pressure in the fuel line. In order to control such a gas engine, for example when the load varies, the pressure in the fuel line must consequently be controlled by a governor. However, such control is expensive and under certain conditions causes irregular combustion and unstable and unreliable operation of the gas engine, which in turn can lead to operational and mechanical problems. Another problem of such mechanical control of a two-stroke gas engine is that unburnt fuel from the cylinder can flow into the intake manifold and/or exhaust pipe when the speed of the gas engine is suddenly reduced, which can lead to dangerous explosions in the intake manifold or exhaust pipe. In addition, mixture is consequently forced back into the gas system, leading to poor metering accuracy and thus misfiring. Hence, when gas engines, especially large-bore gas engines, are run at less than full load, they tend to mis-fire due to the air fuel ratio be too lean for consistent light-off by the spark plug. This is particularly true with pump or blower scavenged engines. These engines have an air pump (or blower) that is driven at engine speed, or a multiple of engine speed, i.e. the amount of air that is pushed through the engine is a function only of the engine speed. These air supplies tend to continue to put a large amount of air in the cylinder (enough for full load). When load is reduced, to maintain a constant speed, the fuel is cut back by the governor to prevent over-speed, which results in an overly lean mixture in the cylinder. Since the air is constant and fuel is reduced, this has a profound effect on the trapped air fuel ratio that the cylinder sees for combustion. This overly lean mixture is hard to ignite, and soon the engine is in lean mis-fire, which causes unburned fuel to be exhausted out the tailpipe of the engine.
In order to avoid these problems, individually controlled gas valves, e.g., hydraulically and/or electromagnetically controlled valves which inject a predeterminable gas amount into the cylinder during the intake stroke, can be employed instead of the mechanical fuel feed valves. However, in order to be able to feed sufficient fuel into the cylinder despite the very short injection times and the small available opening cross sections, fuel at high pressure is required in these systems, which increases expenditures. Such a gas valve results is disclosed in AT 413 136 B, for example.
A known method of controlling the speed of a gas engine with individually controlled gas valves operating at loads of less than their rated load is to skip a number of cylinders, e.g. one or more of the number of cylinders, every revolution (or every two revolutions in case of four stroke engine) of the crank shaft by the control system. Every revolution the cylinder(s) skipped is changed. This means that not all cylinders are fuelled but only the number of cylinders required for the current load. In this mode the control system would automatically skip a cylinder because the load did not need all the power the engine was capable of producing, and by sending no fuel to a cylinder which would have mis-fired anyway, the fuel can be saved.
It is likewise possible in four-stroke engines to feed the gaseous fuel directly into the intake line in which the mixing of air and gaseous fuel then takes place. However, ignitable mixture is then present in the intake manifold, which can lead to undesirable backfiring.
A spontaneously igniting gas engine is disclosed in JP 08-028 268 A, where through a controlled valve a defined gas amount is introduced into an auxiliary combustion chamber. At the end of the compression phase a mechanical valve is opened so that hot compressed air is able to flow into the auxiliary combustion chamber through which the gas mixture present in the auxiliary combustion chamber is ignited. The ignited gas mixture then expands into the cylinder and brings about the power stroke. However, nothing with regard to metering of gaseous fuel directly into the cylinder of a spark-ignited gas engine may be deduced from JP 08-028 268 A.
It is an object of the invention to provide a spark-ignited gas engine and a method for controlling such a gas engine which allows accurate, flexible metering of gaseous fuel into the cylinder and hence accurate, flexible control of the gas engine, even at loads of less than their rated load.